With the rapid development of modem power electronic technology and microelectronic technology, high-voltage and high-power frequency converter have gradually matured. Taking a high-voltage and high-power frequency converter as an example, in addition to inclusion of a rectifier circuit, an inverter circuit, a filter circuit and other parts, a DC (Direct Current) bus needs to be equipped with a precharge device. The precharge device is used to limit current charging a capacitor at the moment of the power source being powered on, such that elements of the converter can be protected from being damaged due to an instantaneous short-circuit current of the capacitor.
The voltage of the DC bus in the high-voltage and high-power frequency converter may be as high as tens of thousands volts, so insulation of the precharge device is a difficult problem. Moreover, in a multi-level circuit including a flying capacitor, the precharge voltages required by individual capacitors are different, resulting in problems of cost, volume and complexity.
The above problem is a challenge for frequency converters which require high efficiency, high power density, high reliability and low cost.
It should be noted that, the above information disclosed in this Background section is only for helping understanding of the background of the present disclosure, therefore, it may include information that does not constitute prior art known by those skilled in the art.